Aquila X-1 returns to quiescence : optical monitoring of the 2021 outburst

ATel #14563; Payaswini Saikia, David M. Russell, Maria Cristina Baglio, D. M. Bramich (NYU Abu Dhabi) and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 20 Apr 2021; 08:13 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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The transient neutron star low-mass X-ray binary Aquila X-1 (Aql X-1) is known to have frequent outbursts every year, with each outburst typically lasting from a few weeks to a few months (e.g. Degenaar et al. 2019). The most recent outburst of the source was reported by the Zwicky Transient Facility (ZTF; Bellm et. al 2019, Graham et. al 2019), when they detected the source on MJD 59267 (2021 February 22) with an optical ZTF-r filter magnitude of ~17.6 (ATel #14437).

Here we report on the optical monitoring of Aql X-1 during the 2021 outburst, and the recent optical fading of the source back to quiescence. We have been frequently monitoring Aql X-1 since 2006 April with the Las Cumbres Observatory (LCO) 2-m (Faulkes) and 1-m robotic telescopes (see e.g. ATel #1218, #2871, #9306, #13953). We detected the source on MJD 59280.4 (March 7) confirming the outburst seen with ZTF, with optical magnitudes i'=17.42+/-0.02 and V=18.26+/-0.03. Prior to this, the source was detected in quiescence with magnitudes i'=18.17+/-0.02 and 18.67 on MJD 59261 (February 16).

The source rose in all the optical bands towards the peak of the outburst (around MJD 59297-59300), with the brightest magnitudes being '’=16.77+/-0.01 and r' =17.12+/-0.01 on MJD 59297 (March 24); and V=17.67+/-0.02 and R=17.13+/-0.01 on MJD 59300 (March 27). The i'-band data which was closely monitoring the source during the outburst, shows a variability of ~0.3 magnitudes during the peak of the outburst (MJD 59297-59302). This is one of the fainter outbursts of Aql X-1, as normally the outburst peaks are observed at much brighter magnitudes. For comparison, the previous outburst seen in 2020 had peak magnitudes of i'=16.07+/-0.01, R=16.20+/-0.01 and V=16.64+/-0.01 on MJD 59107.9 (2020 Sept 15).

After reaching the peak magnitudes around MJD 59300, Aql X-1 started its decay in all the optical bands. The most recent observations of the source obtained on MJD 59323.1 (2021 April 19), with magnitudes i'=18.07+/-0.03, R=18.37+/-0.02 and V=19.16+/-0.03, are consistent with the source being back in quiescence. We expect the outburst to have ended between MJD 59314-59323, with the last detection during the outburst being on MJD 59314 (April 10) with i' = 17.72+/-0.01 and r'=18.21+/-0.01.

We will continue to monitor Aql X-1 in optical wavelengths as part of an on-going monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. The analysis of the LCO data are performed with a new real-time data analysis pipeline, the ''X-ray Binary New Early Warning System'' (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Aql X-1 optical light curve